An ordinary image processing apparatus has three chips of 2-D image pickup devices such as CCDs, on which additive primary color filters, an R-color filter, a G-color filter and a B-color filter, are attached respectively.
The image processing apparatus employs a multi-chip image pickup method that acquires a piece of full-color image information for one frame by separating an image of a subject input through an optical system at a shoot with a spectral prism, and by launching the separated images onto the three 2-D image pickup devices.
In contrast with this, there is an image processing apparatus including only one 2-D image pickup device that has individual pixels arrayed two-dimensionally, each of which has one of the R-color filter, G-color filter and B-color filter attached to the pixel.
Since this image processing apparatus can obtain only one color signal (one of R, G and B color signals) from each pixel, it employs a single-chip imaging method that acquires full-color image information of the individual pixels in a quasi manner by obtaining unacquirable two color signals by calculations using color signals of the neighboring pixels of the individual pixels.
As compared with the multi-chip image pickup method, the single-chip imaging method can reduce the number of the optical parts of the image pickup device, thereby being able to construct a smaller and cheaper apparatus. Thus, consumer digital still cameras or digital video cameras widely employ the single-chip imaging method.
As described above, the single-chip imaging method acquires a full-color image by generating non-pickup color signals by interpolation of the color signals using image information acquired by the single-chip image pickup device to which the primary color filters are attached. The R, G, B signals of the individual pixels generated by the color interpolation are finally converted into luminance/color difference signals to be subjected to screen display or image compression such as JPEG/MPEG, usually followed by filtering such as noise reduction and contour emphasis of the luminance/color difference signals before the image compression.
In the conventional image processing methods described in the following Relevant Documents 1 and 2, when the image pickup apparatus employs the three-chip image pickup device, a luminance/color difference separation circuit carries out the conversion to the luminance/color difference signals, and when the image pickup apparatus employs the single-chip image pickup device, the luminance/color difference separation circuit carries out the above-mentioned color interpolation, and then the conversion to the luminance/color difference signals.
Then, the conventional image processing methods reduce the noise by performing contour emphasis or noise reduction processing called coring of the luminance/color difference signals converted by the luminance/color difference separation circuit.
As for the conventional image processing method described in the following Relevant Document 3, although it carries out the noise reduction processing during the color interpolation of an image acquired by the single-chip image pickup device, it cannot prevent the noise diffusion caused by the color interpolation because it is only after generating the luminance signal of all the pixels by the color interpolation that the specified low-pass filtering is performed. Furthermore, to configure a pixel window for noise reduction processing of the luminance signal after the color interpolation, an additional line buffer is required.
As for the conventional image processing method described in the following Relevant Document 4, it discloses a technique of carrying out noise elimination processing before performing color interpolation of an image acquired by the single-chip image pickup device. It can reduce noise because the color interpolation does not diffuse the noise.
However, since it carries out noise level detection and correction value calculation using only pixels with the same color component as the pixel of interest in a specified region, it brings about difference in the noise levels, which are detected between adjacent different image acquisition color pixels, particularly at color edges. This deteriorates the continuity of the noise reduction on the screen, and the stability of the image quality. In addition, although it utilizes the noise levels of processed adjacent pixels with the same color recursively during the noise level detection, it has little effect on the reduction of random noise occurring independently of the adjacent pixels. Furthermore, although it decides a region with a high noise level as an edge significant as image information by a fuzzy function, and obviates the noise reduction processing in that region, it cannot reduce noise occurring in the pixels adjacent to the edge, and the noise will be emphasized by the contour emphasis that will be usually used at a post stage.    Relevant Document 1: Japanese patent No. 2787781.    Relevant Document 2: Japanese patent application laid-open No. 2001-177767.    Relevant Document 3: Japanese patent application laid-open No. 2003-87809.    Relevant Document 4: Japanese patent application laid-open No. 2003-153290.
As for the conventional image processing methods with the foregoing configurations, the noise superimposed on the image signal during the photoelectric conversion of the image pickup device, and the noise superimposed on the analog signal after the photoelectric conversion (noise produced by an analog signal processing circuit) are diffused to the neighborhood of the pixel of interest by the color interpolation. Consequently as the number of pixels of the image pickup device increases, and as a receiving area per element decreases, the sensitivity reduces, which presents a problem of unable to reduce the relatively increasing noise sufficiently.
More specifically, although the luminance noise can be reduced by the coring and the color noise can be reduced by the low-pass filtering to some extent, the actual acquired image has random noise over the entire image rather than spot noise. Accordingly, the noise diffused by the color interpolation overlap on each other, and the original image signal is buried in the noise. Thus, it is difficult to remove the luminance noise or color noise after converting to the luminance/color difference signals.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide an image processing method capable of preventing the noise superimposed on the pixel of interest from being diffused to the neighboring pixels, and capable of reducing the noise sufficiently.